Weight versus Density Altitude

Recently I had my Biennial Flight Review (BFR) at Minuteman Jet Center in Missoula, Montana. I’ve been flying for 11 years, and yet Art always has something new and great to teach me. (A good pilot is always learning! – If you need a BFR in Missoula, call Minuteman!) This flight was no exception. Art asked me to look carefully at the Takeoff Distance performance charts for the planes I usually fly, Cessna 172 and 182s. There’s something fairly innocuous and well-hidden in them. It seems that weight is more important than Density altitude with respect to takeoff performance. What?

Mountain flying courses always advocate early or late departures, when the air is cool and the turbulence is low. Yes, that’s a great idea! However, when you do the math, as I present below, the numbers pan out: Weight is relatively more important than Density altitude when it comes to takeoff performance.

Let’s look at different temperatures and weights for a popular backcountry (“bush”) airplane, the 1981 Cessna 182 Turbo Skylane.

Graphic comparing weight versus temperature

Density altitude
What is Density altitude? This is the term for pressure altitude that has been corrected for nonstandard temperature. Basically, the more hot and humid the air is, the thinner the air, which results in decreasing aircraft performance. It is most pronounced in naturally-aspirated, or non-turbo aircraft. Air naturally gets thinner the higher you go, so when you combine high altitudes (over 3,000 feet MSL in aviation parlance) with high temperatures, the effect is exacerbated. The airplane performs like it is at an altitude much higher than it actually is, increasing takeoff and landing distances, and decreasing climb rates. (Click on the links in gray to learn more about these subjects.)

Takeoff performance at different temperatures
In the table below, I compare takeoff distances at a Pressure Altitude of 4,000 feet. (All distances in this discussion are to 50 feet Above Ground Level [AGL], see notes at the bottom of this post for specifics and conditions).

T182 Takeoff Distance (to 50 feet AGL) at 4000 feet pressure altitude

Weight

32°F(0°C)

50°F (10°C)

68°F (20°C)

86°F (30°C)

104°F (40°C)

3100 pounds

1670

1815

1970

2145

2345

2800 pounds

1320

1430

1550

1675

1815

2500 pounds

1030

1110

1195

1290

1395

Note, that at Gross weight (3100 pounds), At 86 degrees, it takes 2145 feet of runway to reach 50 feet AGL. Reducing the weight of the aircraft by only 10%, to 2800 pounds, decreases the takeoff distance by 22%, or 1675 feet. To get the same performance (22% reduction in runway required) at Gross weight by only waiting for the temperature to drop, you would need the temp to decrease to freezing (32 degrees). At 32 degrees, the takeoff distance is only 5 feet shorter, at 1670 feet. Here, you can see that offloading 300 pounds has the same effect as a temperature drop of 54 degrees!

At Gross, reducing your load by 10% (300 pounds) has the same effect on takeoff performance as waiting for the temperature to drop from 86 to 32 degrees!! Lighten that load!

However, most pilots won’t want to wait until the temperature is freezing, and in some places in the mountains it may be well after dark before that happens. A lot of backcountry strips are not safe to depart after dark, what with no external lights, canyons, mountains, and trees in the flying corridor. Chances are, you will depart at dusk at the latest, to climb well above the terrain to MEF/MEAs. Let’s consider a more realistic temperature – 50 degrees at departure. At Gross, waiting for the temperature to decrease to 50 degrees F will only afford you a 15% decrease in runway required.

In the table below, I compare the percent reduction in takeoff distance compared to Gross weight at 86 degrees. (I don’t use 104 degrees in my examples because it is unrealistic in mountain environments – if it that hot, you’ll want to wait until it is way cooler to depart!)

Or, here’s another way to look at it – Compared to the runway required at 3100 pounds and 86°F, what percent is required at lower weights and temperatures? Again, note that lightening the load by 300 pounds (10%), has the same effect as waiting for the temperature to drop to 32 degrees!

Conditions: Flaps at 20 degrees, 2400 RPM and 31 inches Hg prior to brake release. Mixture full rich, Cowl Flaps open, Paved, Level, Dry Runway, Zero Wind. Increase distances by 15% for a dry, grass runway for “ground roll” figures (not listed in this report.) However, on cursory examination of the data, the percentages listed are +/- 2% when considering the takeoff roll data, and therefore are relative to this report.

Disclaimer: You must consult the Operator’s Manual in your aircraft to assess takeoff requirements. I assume no liability for the use of this information whatsoever. The examples given are best-conditions, and the numbers apply to a paved, level runway and not to grass, dirt, or primitive strips. There is no substitute for proper training, supervision, and experience. Mountain flying can be deadly to the ill-prepared. Is this enough disclaimer?

Feel free to download the graphic below (as long as you maintain author attribution)